Earth's core rotating faster than rest of the planet but slower than previously believed

Feb 20, 2011
Earth

New research gives the first accurate estimate of how much faster the Earth's core is rotating compared to the rest of the planet.

Previous research had shown that the Earth's core rotates faster than the rest of the planet. However, scientists from the University of Cambridge have discovered that earlier estimates of 1 degree every year were inaccurate and that the core is actually moving much slower than previously believed – approximately 1 degree every million years. Their findings are published today, Sunday 20 February, in the journal Nature Geoscience.

The inner core grows very slowly over time as material from the fluid outer core solidifies onto its surface. During this process, an east-west hemispherical difference in velocity is frozen into the structure of the inner core.

"The faster rotation rates are incompatible with the observed hemispheres in the inner core because it would not allow enough time for the differences to freeze into the structure," said Lauren Waszek, first author on the paper and a PhD student from the University of Cambridge's Department of Earth Sciences. "This has previously been a major problem, as the two properties cannot coexist. However, we derived the rotation rates from the evolution of the hemispherical structure, and thus our study is the first in which the hemispheres and rotation are inherently compatible."

For the research, the scientists used seismic body waves which pass through the inner core - 5200km beneath the surface of the Earth - and compared their travel time to waves which reflect from the inner core surface. The difference between the travel times of these waves provided them with the velocity structure of the uppermost 90 km of the inner core.

They then had to reconcile this information with the differences in velocity for the east and west hemispheres of the inner core. First, they observed the east and west hemispherical differences in velocity. They then constrained the two boundaries which separate the hemispheres and found that they both shifted consistently eastward with depth. Because the inner core grows over time the deeper structure is therefore older, and the shift in the boundaries between the two hemispheres results in the inner core rotating with time. The rotation rate is therefore calculated from the shift of the boundaries and the growth rate of the inner core.

Although the inner core is 5200km beneath our feet, the effect of its presence is especially important on the Earth's surface. In particular, as the inner core grows, the heat released during solidification drives convection in the fluid in the outer core. This convection generates the Earth's geomagnetic field. Without our magnetic field, the surface would not be protected from solar radiation, and life on would not be able to exist.

"This result is the first observation of such a slow inner core rotation rate," said Waszek "It therefore provides a confirmed value which can now be used in simulations to model the convection of the Earth's fluid outer core, giving us additional insight into the evolution of our magnetic field."

Explore further: Tropical depression 21W forms, Philippines under warnings

More information: The paper 'Reconciling the hemispherical structure of Earth's inner core with its super-rotation' is scheduled for advanced online publication in Nature Geoscience on Sunday 20 February.

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User comments : 17

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rodgod
1 / 5 (5) Feb 20, 2011
So, I'm guessing this slowing is preemptive to the reversing of the polarity of the magnetic poles? Anybody care to guess about the timeline for such a reversal?
VitalStatistic63
4.4 / 5 (7) Feb 20, 2011
I didn't read from this article that the core was slowing, although it most likely is. I read that previous research was inaccurate and the core speed estimate has been adjusted to suit.

Your post implies that pole reversal is a result of the direction the core is spinning, and therefore that it has changed direction many times in the past. I can think of no mechanism that would cause the core to actually change direction of rotation. You have to remember, there is no down direction at the centre of the Earth. The biggest gravity influence on our core is the Sun. That may cause an irregularly shaped core to have a heavy side that is swinging like a pendulum and depending on which way it currently hangs, could influence the direction of the magnetic field.

The core rotation would be slowing due to friction within the mantle, and eventually it will stop altogether, if not due to friction, then due to the cooling and solidification of the mantle.
astro_optics
2 / 5 (8) Feb 20, 2011
The Global Warming Alarmists will be pleased to know that this is anthropogenic magnetic pole reversal :P
StillWind
1 / 5 (1) Feb 20, 2011
The core rotation would be slowing due to friction within the mantle, and eventually it will stop altogether, if not due to friction, then due to the cooling and solidification of the mantle.


The mantle is solid. It is the outer core that is theorized to be some form of liquid, although other theories say that it is not.
OTOH, you are correct in pointing out that there is no know mechanism that would change the rotation of the core, making polar shifts all the more interesting.
In fact, polar shifts are a good reason to discount the current model, altogether.
rwinners
not rated yet Feb 20, 2011
>The core rotation would be slowing due to friction within the mantle, and eventually it will stop altogether, if not due to friction, then due to the cooling and solidification of the mantle.<

I presume you mean in relation to the crust, but then, why hasn't it syncronized already?
nuge
5 / 5 (1) Feb 20, 2011
rwinners, you are supposed to use [ q ] QUOTATION [ / q] (without the spaces) in order for quotes to appear like this:
QUOTATION
rwinners
not rated yet Feb 20, 2011
What, was my post not understandable?
nuge
5 / 5 (1) Feb 20, 2011
No, I was just trying to help. Sorry if that offended you.
Quantum_Conundrum
1 / 5 (4) Feb 21, 2011
I presume you mean in relation to the crust, but then, why hasn't it syncronized already?


Because the idjits here are wrong.

The core is very dense iron, nickel, and heavy radioactive metals. Because of conservation of angular momentum, this means the core spins faster than the less dense crust and mantle.

Friction cannot change the angular momentum of the Earth-moon-sun system, nor any individual member in the system.

If the core slows down, the crust of the earth as well as the moon's orbit must speed up simultaneously in order to compensate for this change in angular momentum, until they all become gravitationally locked, or until the moon flies off into space.

Now due to tidal forces, the crust of the earth slows down as the moon speeds up. HOwever, due to other forces, the core of the earth slows down while acting as an accelerating force on the crust, offseting some of the moon's tidal influence.
rwinners
5 / 5 (2) Feb 21, 2011
Ok, but over time - stellar time - the earth's revolution has slowed down and the moon's orbit has moved away from the earth. What's been happening to the core? By your logic, it should have speeded up?
frajo
5 / 5 (1) Feb 21, 2011
No, I was just trying to help. Sorry if that offended you.
I'm always puzzled, too, why some people don't just use the "quote' button.
VitalStatistic63
not rated yet Feb 21, 2011
The mantle is solid. It is the outer core that is theorized to be some form of liquid, although other theories say that it is not.

I might be thinking of the wrong bit. I was under the impression there was a solid core and a solid crust with a liquid section of melted rock in between. I thought that was called the mantle. Basically the crust floating on a sea of lava.
VitalStatistic63
5 / 5 (1) Feb 21, 2011
Ok, but over time - stellar time - the earth's revolution has slowed down and the moon's orbit has moved away from the earth. What's been happening to the core? By your logic, it should have speeded up?

No. Gravitational effects from the moon are slowing the whole Earth down, the crust, the core, the oceans, the whole lot. This is also causing the Moon to speed up and pushing it's orbit outwards.

Seperately to that, the core is dragging on the crust causing them both to move towards the rotation speed of the other, thus causing the core to slow down while the crust speeds up.

As well as that, eventually we will become tidally locked to the sun and rotate only once per year, the same as the Moon is now tidally locked to the Earth.
antialias
not rated yet Feb 21, 2011
As well as that, eventually we will become tidally locked to the sun and rotate only once per year, the same as the Moon is now tidally locked to the Earth.

Unlikely, as the sun will swallow the earth way before then.
Quantum_Conundrum
1 / 5 (3) Feb 21, 2011
Ok, but over time - stellar time - the earth's revolution has slowed down and the moon's orbit has moved away from the earth. What's been happening to the core? By your logic, it should have speeded up?


No, the core is influenced tidally by the moon as well, just not as much.

As the moon speeds up, both the core and crust of the earth slow down.

As the core slows down, the crust has a certain element which wants it to speed up, but this is totally masked by the stronger tidal forces.

The crust and core "should" eventually converge on the same rate of rotation.
rwinners
not rated yet Feb 21, 2011
The mantle is solid. It is the outer core that is theorized to be some form of liquid, although other theories say that it is not.

I might be thinking of the wrong bit. I was under the impression there was a solid core and a solid crust with a liquid section of melted rock in between. I thought that was called the mantle. Basically the crust floating on a sea of lava.

trucksmart
not rated yet Feb 22, 2011
The part I received as most believable is that the earth's diameter continues to grow. Even with erosion, the matter solidifying is expanding matter and causes the earth's surface to grow which in turn slows down the earth's rotation more each century. The same way a ballerina slows as their arms extend while spinning.

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